The present invention relates to a metal covered polyimide composite for use as a mounting material of electronic components such as a flexible print substrate, TAB and COF (Chip on Film), and additionally relates to a process for producing the composite and a process for producing an electronic circuit board.
An FCCL (Flexible Copper Clad Laminate), in which metal conductor layers primarily formed from copper are laminated on a polyimide film, is broadly used as a circuit board material in the electronics industry. Among the above, a non-adhesive flexible laminate (especially a two-layer flexible laminate) that does not include an adhesive layer between a polyimide film and a metal layer is catching attention on the demands of finer pitches of the circuit wiring width.
As a method of producing a non-adhesive flexible laminate; in particular a non-adhesive flexible laminate capable of accommodating fine pitches, primarily performed is a so-called metalizing method of forming, in advance, a tie-coat layer configured from a material having favorable adhesion with polyimide on a polyimide film by a drying method such as sputtering, CVD, or vapor deposition and a metal seed layer as a conductor of the cathode/current in the subsequent electroplating process, and thereafter forming a metal layer film to become the conductor layer of the circuit board by way of electroplating (refer to Patent Literature 1).
In this metalizing method, for better adhesion strength between the metal layer and the polyimide film, modification is carried out by performing plasma treatment to the polyimide film surface to eliminate the contaminants on the surface and to improve the surface roughness prior to forming the metal layer (refer to Patent Literature 2 and Patent Literature 3).
Generally, when pre-forming a metal layer on a polyimide film by a drying method such as sputtering, the improvement of adhesion and etching properties is being sought through the selection of the interlayer material (refer to Patent Literature 4).
In addition, proposed is a polyimide film with a metal film for use in TAB or FPC obtained by performing chemical etching to the polyimide film surface so to roughen the surface, forming a foundation layer thereon, and additionally forming a copper vapor deposition layer thereon (refer to Patent Literature 5).
When a metal covered polyimide composite is to be used as a mounting material of electronic components such as a COF (Chip on Film), the metal layer on the polyimide is partially eliminated to create a circuit pattern, tin plating is subsequently performed on the copper layer forming the circuit pattern, and treatment such as solder resist or resin seal is additionally performed on the tin plated layer. However, a problem in that this tin plated layer may peel occurs. A major cause of such peeling is the Kirkendall voids (air gaps) generated between the copper layer and the tin plated layer due to the electroplating process. The Kirkendall voids will be explained in detail later.
An electroplated copper layer is usually formed with a plurality of electrolytic baths, and the electroplating current conditions of the copper layer will change considerably among the electrolytic baths as a matter of course. The portions in which the electroplating current conditions change considerably will become copper plated layer boundaries where impurities will increase and be concentrated easily. Since the Kirkendall voids particularly occur at the portions where the copper plated layer boundary and the tin plated layer are adjacent to each other, if plating is to be performed in multiple electroplating baths, then the Kirkendall voids will arise at least in a quantity that is equivalent to the number of electroplating baths.
As a method of overcoming the foregoing problem, proposed is a method of forming a copper layer from the surface layer to an area that is at least three times the tin plated layer using the same electrolytic bath in a copper plating film to be formed using a plurality of electrolytic baths (refer to Patent Literature 6). Patent Literature 6 has analized that the Kirkendall voids causes the peeling.
Nevertheless, in the foregoing case, Patent Literature 6 only sees the tin covered on the uppermost layer of the copper plating as the problem. However, when forming a copper circuit and covering it with a tin layer, the tin layer will be deposited on the side face in addition to the uppermost layer of the copper. In addition, as described later, since a copper layer boundary may occur even with the same electrolytic bath, it cannot be said that the foregoing method is a sufficient solution to the problem.
Accordingly, the problem of Kirkendall voids arising in the bonded interface of the multiple copper layers (nine layers in the Examples of Patent Literature 6) and the tin layer has not been resolved. In addition, with a copper layer in which only the uppermost layer is thick, the other copper layers must be made thin by just that much, and there is a problem in that the balance of the copper layers will be lost.
In light of the above, a zigzag-type electroplating process that requires numerous electroplating baths is undesirable, and it is preferable to reduce the number of electroplating baths as much as possible. In order to obtain a plated copper layer of the intended thickness with few electroplating baths, it is necessary to raise the plating current density, and in this respect a drum electroplating method is effective. The drum electroplating method is feeding a polyimide film, in which a tie-coat layer and a metal seed layer are formed thereon by electroless plating or a drying method, around a drum surface that is dipped in an electrolyzer, and thereby performing copper plating to the surface. Since this method enables to move the anode and cathode closer without having disturbances such as the warping or blurring during the feeding of the film to become the cathode and to control the distance constant between the anode and cathode, it is possible to increase the current density.
Moreover, as a result of moving the anode and cathode closer together, the flow velocity of the plating electrolytic solution can also be increased easily, and this is also effective in increasing the current density. Accordingly, the drum electroplating method is effective in preventing the increase and concentration of impurities, and thereby decreasing the incidental Kirkendall voids.
Nevertheless, this drum electroplating method also entails its own problems. It is necessary to increase the current density in order to increase the electrodeposition rate of the electroplating process. However, at the initial stage of starting the electroplating process, the metal seed layer formed on the polyimide surface is unable to withstand a large current since its thickness is limited. Consequently, the anodes set to face the drum are divided into a plurality of zones, and the current density to the respective zones is being controlled independently.
As a result, each time the amount of feed (amount of current) to the anode changes, a different copper plated layer is formed, and the impurities will increase and be concentrated at the copper plated layer boundary, whereby the Kirkendall voids will arise. Conventionally, in consideration of the production efficiency, a feeding method of disposing numerous anodes and dividing the zone into four zones or more is being adopted, and, as a result, the number of copper layers will be four or more.
In addition, when feeding the plating material around the drum and performing plating upon providing anodes at the opposing position, a proposal has been made of making the space between one anode and the plating drum to be different for each plating area, or disposing a differently sized mesh between the anode and the plating drum (refer to Patent Literature 7). It may appear that the increase of impurities and the generation of the Kirkendall voids can be inhibited by reducing the number of plating baths.
However, this idea is unrealistic since it merely proposes the use of one anode because the control of the current density for each anode is troublesome. In other words, besides the supply method being unclear, the flow velocity of the plating solution will be disturbed and it will not be possible to ensure a uniform flow velocity if mesh or the like is disposed. Specifically, with Patent Literature 1, mesh or the like will become an impediment whereby the flow velocity of the plating solution cannot be controlled, and it will be difficult to ensure the uniformity of the amount of electrodeposition in a direction that is parallel to the rotational axis of the drum. Aside from the problem of Kirkendall voids, this idea relates to the problem of plating uniformity, so it could be said that it is an unrealistic method. In addition, this technology fails to even recognize the problem of Kirkendall voids.
Accordingly, it could be said that the conventional technologies fail to teach a fundamental solution to the problem of Kirkendall voids that arise between the copper layer and the tin layer.
[Patent Literature]
    1. JP Patent No. 3258296    2. JP Patent No. 3173511    3. PCT (WO) 2003-519901    4. Japanese Patent Laid-Open Publication No. H6-120630    5. Japanese Patent Laid-Open Publication No. H6-210794    6. Japanese Patent Laid-Open Publication No. 2007-214519    7. Japanese Patent Laid-Open Publication No. 2007-204848